In the production of screeds, inorganic binders, such as natural or synthetic anhydrite, gypsum or cement, partially unleaned, but generally in conjunction with aggregates (leaning agents), such as sand and gravel, are generally mixed and processed, optionally using additives, such as plasticizers.
German published application 2,657,252 describes a process for the production of a free-flowing composition of anhydrite, an activator, a sulfite- or sulfonic-acid-modified resin based on an amino-s-triazine containing at least two NH.sub.2 groups as plasticizer, nonylphenol hexaglycol ether as wetting agent and water. The resin content is from 0.01 to 0.5% by weight and preferably from 0.2 to 0.4% by weight, based on the binder. The water-to-solids ratio shown in Examples 1 to 3 is approximately 0.3 to 0.35. It is also apparent that the anhydrite used is finely ground anhydrite binder. Anhydrite AB 200 according to German Industry Standard ("DIN") 4208 (October 1962 version), new name anhydrite AB 20 according to DIN 4208 (March 1984 version), is used. No leaning agent is used.
The screed produced by this process has a number of disadvantages. As can be seen from "Zement-Kalk-Gips" 17 (1964) 163-466, screeds of finely ground, unleaned anhydrite binder show a tendency towards cracking and warping on account of the excessive shrinkage and swelling.
This tendency to crack can normally be prevented by addition of leaning agents, such as sand, to the mortar mixture. In addition, the high water-to-solids ratio of approximately 0.3 to 0.35 leads to low strength and low setting times.
German 1,297,563 describes the use of anhydrite having a particle size distribution of 0 to 6 mm with at least 30% of the particles smaller than 0.2 mm, sulfate-based activator and water for the construction of face-roadway fills and permanent support piers in mining. The water-to-anhydrite ratio is 0.10 to 0.16 and more especially 0.12 to 0.14. During hardening, the anhydrite increases in volume as, therefore, do the components made from the described anhydrite, which can lead to cracking and warping.
Now, the object of the present invention is to provide an anhydrite-based self-levelling (i.e. the mixture automatically forms a horizontal surface under the effect of gravity, so that no further measures have to be taken before or after hardening of the mxiture to establish a horizontal surface) mortar mixture which does not have any of the disadvantages mentioned above.